1. Field of the Invention
The present invention relates to a technique of detecting a standing wave in a room.
2. Description of the Related Art
When a sound source such as a speaker generates a sound in a room of, e.g., a home, not only direct sounds that reach various points of the room at minimum distances but also reflected sounds from various planes such as the walls, ceiling, and floor of the room are generated. These sound waves overlap each other. If a composite wave formed by the overlapping waves moves neither forward nor backward, and the maximum amplitude of the composite wave at each point is determined not by time but only by its position in the space, the composite wave is called a standing wave.
Especially, the standing wave is readily generated between planes opposing each other at a frequency at which the distance between the planes is an integer multiple of the half wavelength of the sound wave. At this time, the positions of the walls correspond to the anti-nodes of the composite wave. In addition, since sound waves are generally hardly attenuated in a lower frequency range, standing waves are easily generated in a lower frequency range.
The sound of a frequency at which a standing wave is generated becomes too loud and booms at the position of a peak (anti-node), or is conversely hard to listen at the position of a dip (node), resulting in serious problems for human audibility. Hence, when a user wants to enjoy music from a speaker in a room, it is important to grasp the state of each standing wave generated in the listening area and, more particularly, the extreme points such as the peak and dip of each frequency component. Accurately grasping the extreme points of standing waves enables to cope with the standing waves by effectively using the information, for example, correct the sound field to suppress the standing waves or recommend an appropriate listening point.
Conventionally, to know the states of standing waves in a listening area, fixed-point measurement is conducted in general at several discrete points in the listening area, including a point in the listening area which is regarded by the user as the most important listening point. More specifically, a microphone is installed on a tripod or the like at each measurement point. The dip frequency (or peak frequency) of the measured frequency response is detected as the frequency at which a standing wave is generated. However, since a standing wave whose maximum amplitude is determined by its position in the space inevitably has position dependence, it is very difficult to detect the frequencies of all standing waves generated in the listening area based on the fixed-point measurement results at several discrete points. FIG. 19 is a graph showing an example of a result obtained by performing fixed-point measurement at 21 points at an interval of 10 cm on a 2-m direct path in an area assumed to be a listening area in an actual room. Each line of the graph corresponds to the result of one measurement point. For example, frequency components indicated by the arrows largely change the sound pressure levels depending on the measurement point, and are therefore supposed to be strong standing waves having high position dependence. When measurement is performed at discrete fixed points, and a measurement result (line of the graph) corresponding to a selected measurement point is the lowermost line at the frequency at which a standing wave is generated, the frequency is detected as a dip frequency (or if the line is uppermost, the frequency is detected as a peak frequency). In FIG. 19, however, if the line is almost at the center of the arrow, it is not recognized as a standing wave.
For the above-described reasons, to accurately grasp the states of standing waves with high position dependence, it is necessary to repeatedly perform fixed-point measurement at a fine measurement point interval. However, the fixed-point measurement at a fine measurement point interval increases the load on the user and requires long time. Japanese Patent Laid-Open No. 4-93727 proposes a method of introducing a mechanism for controlling a microphone position using a traversing device for automatic measurement.
There is also a method of calculating the frequency of a standing wave not by measurement but using theoretical expressions or simulations. The theoretical expression of a normal vibration mode frequency in a rigid rectangular parallelepiped room is given byf=c/2·√{(x/L)2+(y/W)2+(z/H)2}  (1)where f is the normal vibration mode frequency, c is the sound velocity, L, W, and H are the length, width, and height of the room, respectively, and x, y, and z are integers of 0 or more that specify the mode. However, since equation (1) is used to calculate the frequency, the strength of a standing wave such as the difference between the peak value and the dip value of each frequency component cannot be detected. In addition, since equation (1) assumes an ideal, rigid rectangular parallelepiped room, a deviation from the theoretical expression is generated depending on the actual room structure, the sound absorption characteristic of the walls, and the object layout. A sound field simulation method based on geometrical acoustics, wave acoustics, or the like may be applied in consideration of these conditions. However, considering the labor required for modeling, consistency with actual measured values is not sufficient. Japanese Patent Laid-Open No. 2007-158589 discloses a method of determining the frequency of a standing wave by combining fixed-point measurement and the theoretical expression of the normal vibration mode frequency and collating the dip frequency of a frequency response measured at one point with the theoretical expression.
However, the conventional standing wave detection method has the following problems. FIG. 10 shows another example of the measurement result in an actual room. As shown in FIG. 10, even in the same room, the frequency response largely changes, and the dip frequency varies depending on the measurement point. This indicates that it is very difficult to detect, based on the measurement result at a given point, the frequencies of all standing waves actually generated in the entire room. That is, the method of determining the frequency of a standing wave based on the dip of a measured frequency response allows to detect only the standing wave of a frequency corresponding to the node at the measurement point. In addition, the dip of the frequency response is not necessarily generated only by the standing wave, and a determination error is also possible. On the other hand, the method of calculating the frequency of a standing wave using the theoretical expression of the normal vibration mode frequency in a room assumes an ideal, rigid rectangular parallelepiped room. For this reason, a deviation from the theoretical expression is inevitably generated depending on the actual room structure, the sound absorption characteristic of the walls, and the object layout.